Please I want to update my iOS version

To update to the maximum of iOS 12.5.7, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

iPhone 6 cannot install iOS 15 if that is what you are looking for.

The best that the iPhone 6 can do is iOS 12.5.7